Clostridial Glycine Reductase is an enzyme complex comprised of three protein components: selenoprotein A, a heat stable 12,000 M.W. protein that contains a selenocysteine residue; protein B, a protein of ca. 200,000 M.W. that contains essential carbonyl groups C, ca. 250,000 M.M. Clostridium sticklandii and Clostridium purinolyticum, a purine fermenting organism, were used as enzyme sources. Pure 75Se-label selenoprotein A preparations were isolated from both of these organisms. Small differences in acidity and primary amino acid compositions between the two protein A preparations were observed but the amounts of selenocysteine and cysteine were indentical. Sequence analysis of peptides generated from the selenoproteins from these two sources show that the selenocysteine residues are internal and near the carboxy termini. The phases of this work already completed are basic to eventual analysis of the DNA that specifies the amino acid composition of this small selenoprotein and thus establish whether selenocysteine is the result of a post-translational modification of an amino acid specified by the genetic code. To date the mechanism of insertion of an essential selenocysteine residue in a variety of selenoproteins is completely unknown. The final purification by methods amenable to large scale work of the other two glycine reductase protein components has progressed and is now in final stages of development. Seleno-tRNAs isolated from C. sticklandii and also from a methane producing organism, Methanococcus vanielii, were analyzed for amino acid acceptance activities and also for selenonucleoside contents. In addition to the already identified selenonucleoside, 5-methylaminomethyl-2-selenouridine, two related but different selenonucleosides were detected. A project having to do with the occurrence of selenomethionine in two enzymes present in clostridium kluyveri was finished for the present time since this selenoamine acid was found to be inserted in the proteins as a non-specific substitute for the numerous methionine residues present in the enzymes, thiolase and beta-hydroxybutyrul-coenzyme A dehydrogenase.